source.js

var LocationSource = {
  fetchLocations(country) {
    return {
      remote() {
        // return a promise fetching locations for the given country
        return new Promise(/* do stuff here */).then(
          function(response) {
            return [country, response]
          },
          function(error) {
            return [country, error]
          })
      },

      local() {
        // Never check locally, always fetch remotely.
        return null;
      },

      success: LocationActions.updateLocations,
      error: LocationActions.locationsFailed,
      loading: LocationActions.fetchLocations
    };
  }
};